Whooping Cough Epidemic Hit Vaccinated Kids Hard

Researchers found Tdap vaccine effectiveness fell off after 2 to 4 years.

Action Points

Note that this epidemiologic case-control study of pertussis found that vaccination was not entirely effective at preventing the disease.

Be aware that disease severity was not captured -- vaccines may have ameliorated symptoms without nullifying transmission.

The 2012 pertussis epidemic in Washington state affected a surprising number of adolescents who were vaccinated on schedule, researchers reported.

Compared with over 1,200 controls, 450 adolescent cases of pertussis showed similar rates of having received five rounds of their vaccinations on the recommended schedule, but the vaccine's effectiveness rate dropped dramatically over time from 73% within 1 year to as low as 34% after 2 to 4 years from the last dose, Anna M. Acosta, MD, of the CDC, and colleagues, reported in Pediatrics.

The pertussis vaccine and its schedule have been through several changes in the past 2 decades. Through 1997, children received five doses of a whole-cell vaccination. After 1997, those five doses were switched to an acellular pertussis vaccine. And in 2005, a sixth dose of the tetanus, diphtheria, acellular pertussis (Tdap) vaccine was recommended.

Acosta's team noticed an unexpectedly high rate of infection among older children and adolescents -- those potentially exposed to either a mix of whole-cell and acellular vaccines, or acellular vaccines only.

For this study, the researchers looked at Washington residents born from 1993 to 2000 in the seven Washington counties that reported more than 50 cases, and matched the adolescent pertussis cases with three controls based on vaccination records.

The participants were grouped by vaccination years, 1993-1997 for children who received five total vaccines in a mix of whole-cell or acellular form, or 1998-2000 for children who received five total vaccines in the acellular form only.

In order to be considered "complete and on schedule" for vaccination, the child had to have records indicating five total doses of diphtheria-tetanus toxoids-pertussis (DTP) or DTaP parsed as doses one-three before turning 1-year-old, dose four on or after the first birthday, and dose five on or after the fourth birthday, but before turning 7.

Within the seven counties, 959 cases of adolescent pertussis were reported. These cases represented 83% of all adolescent cases in the state, and 73% of all healthcare providers reporting cases of adolescent pertussis.

The primary analysis by Acosta's group included 450 cases and 1,246 age- and provider-matched controls with a median age of 14.

In the cohort, only 74% of cases and 75% of controls had proof of five vaccination doses, and only 60% of cases and 58% of controls received the five doses on schedule.

Tdap was given to 81% of the cases, and 90% of the controls, and overall, 84% of the cohort had been vaccinated again at 11 to 12 years of age.

The researchers estimated that Tdap demonstrated vaccine effectiveness at a rate of 63.9% (95% CI 49.7%-74.1%).

When stratified by time since Tdap vaccination, the effectiveness fell dramatically over time:

When Acosta's group performed subgroup analysis limited to confirmed cases of pertussis -- those with on-schedule only vaccinations -- or by excluding those with suspected cases, the results weren't much different from the findings in the primary analysis.

In the mixed group, 386 cases and 1,076 controls, the overall vaccine effectiveness was 51.5% (95% CI 26.1-68.1). But, the authors wrote, a direct comparison of the mixed group and the acellular only group wasn't possible due to differences in the age and time since vaccination between groups (P<0.0001).

Although they could only confirm the vaccine brands, Boostrix by GlaxoSmithKline or Adacel by Sanofi, in 76% of study participants, the researchers found that Adacel was used more often in adolescents who caught pertussis (P=0.05), even though Adacel was used in only 17% of cases compared with 59% of cases vaccinated with Boostrix.

The study authors hazarded a few guesses as to why vaccine effectiveness fell short. One explanation could be suboptimal, post-vaccination, cell-mediated immunity with the acellular vaccine. Another could be that the vaccine prevented symptoms, but not infection and transmission. Also, there could have been genetic mutations in the pertussis strains that were not accounted for in the vaccine.

Study limitations include small sample size in comparison with the overall epidemic.

None of the authors reported relevant financial relationships with industry.

Accessibility Statement

At MedPage Today, we are committed to ensuring that individuals with disabilities can access all of the content offered by MedPage Today through our website and other properties. If you are having trouble accessing www.medpagetoday.com, MedPageToday's mobile apps, please email legal@ziffdavis.com for assistance. Please put "ADA Inquiry" in the subject line of your email.